Plastic can be a cost-effective material for many applications. For some applications, however, the lower cost plastics such as polyethylene, polypropylene, and polystyrene lack sufficient stiffness. Wood, on the other hand, has high stiffness. But wood of high quality, which is free of knots and has a uniform grain such that it is suitable for use in mass production products, is expensive and, in recent times, a reliable source of such wood has been difficult to find. Plastic has a lower modulus of elasticity than wood, and hence a stiffness which is less than that required for many applications. This is particularly the case for relatively low cost plastics such as polyethylene, polypropylene, and polystyrene. Wood, while having a substantially higher modulus of elasticity than common plastics, is subject to attack by insects, fungus, and mold. Furthermore, wood, because of its tendency to absorb and lose moisture and the resultant dimensional changes, is dimensionally unstable under certain conditions.
Wood fibers and more generally cellulosic materials may be obtained at low cost and are often available as a waste product or readily manufactured from a waste stream.
By combining plastic with cellulosic materials, composite materials have been developed which combine many of the advantages of wood and of plastic while avoiding the disadvantages of either material. The plastic-wood fiber compounds which are sold as a substitute for plastic have stiffness levels approaching or surpassing those of wood while being available at a cost which is comparable or less than that of plastic with similar structural properties. The composite materials are manufactured by mixing finely divided cellulosic material into molten plastic. The mixed plastic is extruded through a die to form pellets similar in shape to the plastic pellets which are sold as the basic material for manufacturing plastic articles. Although a part manufactured of a cellulosic-plastic composite will typically be designed with that material in mind, standard extrusion and injection molding techniques can be used to fabricate parts with cellulosic-plastic composites.
The addition of cellulose to plastic presents a problem if the hot material during manufacture is exposed to moisture and thus caused to absorb undesirable moisture. It is thus critical that the manufacturing process control and eliminate moisture in the final product.
What is needed is a method of manufacturing a plastic cellulosic composite which removes moisture and prevents moisture from being reabsorbed into the finished product.